Curves of growth of spectral lines emitted by a laser-induced plasma:: influence of the temporal evolution and spatial inhomogeneity of the plasma

The curves of growth (COG) of five Fe I lines emitted from a laser-induced plasma, generated with Fe-Ni alloys in air at atmospheric pressure, have been investigated. Spectral lines with different energy levels and line widths, emitted with a broad range of optical depths, have been included in the study in order to check the validity of theoretical models proposed for COG generation, based in the radiative transfer within a plasma in local thermodynamic equilibrium. The COGs have been measured at time windows of 4-5 mus and 15-18 mus. The Stark widths of the Fe I lines have been obtained, and the line widths have been determined by measuring the plasma electron density at the time windows selected. It is shown that at a time window of 4-5 mus, the inhomogeneity of the plasma magnitudes has an important influence on the COGS of intense lines. For this time window, a two-region model of the plasma has been used to generate theoretical COGS that describe satisfactorily the experimental curves of all the lines using a single set of plasma parameters. The results reveal the existence of considerable gradients between the inner and the outer plasma regions in the temperature (9400-7800 K) and in the density of Fe atoms (4 X 10(16)-0.02 X 10(16) cm(-3) for a sample with 100% Fe). On the contrary, at the time window 15-18 mus, at which the plasma has suffered most of its expansion and cooling process, the COGS of all the lines may be described by a single-region model, corresponding to a plasma with uniform temperature (6700 K) and density of Fe atoms (0.06 X 10(16) cm(-3) for a sample with 100% Fe). It is also shown that at initial times, the plasma inhomogeneity has an important effect in the line profiles of intense spectral lines, which are described by using the two-region model of the laser-induced plasma. (C) 2002 Elsevier Science B.V All rights reserved.